Destination: Moon or Asteroid? Part II: Scientific Considerations

Part II: Scientific Considerations

In my last post, I examined some of the operational considerations associated with a human mission to a near Earth asteroid and how it contrasted with the simpler, easier operations of lunar return. Here, I want to consider what we might do at this destination by focusing on the scientific activities and possible return we could expect from such a mission. Some of the operational constraints mentioned in the previous post will impact the scientific return we expect from a human NEO mission.

Asteroids are the left over debris from the formation of the Solar System. Solid pieces of refractory (high melting temperature) elements and minerals that make up the rocky planets have their precursors in the asteroids. We actually have many pieces of these objects now – as meteorites. The rocks that fall from the sky are overwhelmingly from the small asteroids that orbit the Sun (the exception is that in meteorite collections, some come from larger bodies, including the Moon and Mars).

Moreover, we have flown by almost a dozen small bodies, orbited two, impacted one and “landed” on two others. Thousands of images and spectra have been obtained for these rocky objects. The chemical composition of the asteroids Eros and Vesta have been obtained remotely. We have catalogued the craters, cracks, scarps, grooves and pits that make up the surface features of these objects. We have seen that some are highly fragmental aggregates of smaller rocks, while others seem to be more solid and denser. In addition to these spacecraft data, thousands of asteroids have been catalogued, mapped and spectrally characterized from telescopes on the Earth. We have recognized the compositional variety, the various shapes, spin rates and orbits of these small planetoids. We now know for certain that the most common type of meteorite (chondrite) is derived from the most spectally common type of asteroid (S-type) as a result from the Hayabusa mission, the world’s first asteroid sample return.

In short, we know quite a bit about the asteroids. What new knowledge would we gain from a human mission to one?

Although we have (literally) tons of meteorites, extraterrestrial samples without geological context have much less scientific value than those collected from planetary units with regional extent and clear origins. Many different processes have affected the surfaces of the planets and understanding the precise location and geological setting of a rock is essential to reconstructing the history and processes responsible for its formation and by inference, the history and processes of its host planet.

Most asteroids are made up of primitive, undifferentiated planetary matter. They have been destroyed and re-assembled by collision and impact over the last 4.5 billion years of Solar System history. The surface has been ground-up and fragmented by the creation of regolith and some details of this process remain poorly understood. But in general terms, we pretty much know what asteroids are made of, how they are put together, and what processes operate upon their surfaces. True enough, the details are not fully understood, but there is no reason to suspect that we are missing a major piece of the asteroid story. In contrast, planetary bodies such as the Moon have whole epochs and processes that we are just now uncovering – in the case of the Moon, water has been recently found to be present inside, outside and in significant quantity at the poles, relations that have enormous implications for lunar history and about which we were nearly totally ignorant only a couple of years ago.

Most NEOs will be simple ordinary chondrites – we know this because ordinary chondrites make up about 85% of all meteorite falls (an observed fall of a rock from the sky). This class of meteorite is remarkable, not for its diversity but for its uniformity. Chondrites are used as a chemical standard in the analysis of planetary rocks and soils to measure the amounts of differentiation or chemical change during geological processing. In themselves, chondrites do not vary (much) except that they show different degrees of heating subsequent to their formation, but not enough heating to significantly change their chemical composition.

Some NEO asteroids are pieces of bigger objects that experienced chemical and mineral change or differentiation. Vesta (not a NEO, but a main belt asteroid) has reflection spectra similar to known, evolved meteorites, the eucrite group. These rocks suggest that some asteroids are small, differentiated planetoids, having volcanic activity that dates from the very beginning of Solar System history. Moreover, since we have pieces of the Moon and Mars as meteorite fragments, some NEOs may consist of material blasted off these planets. However, given that most NEOs are inaccessible to human missions, the likelihood that we could visit one of planetary derivation is small (curious that the most interesting of the NEOs appear to be those derived from some bigger (planet-sized) object.) In broad terms of meteorite science, multiple small samples from a variety of asteroid types are preferable to many bigger samples of a single specimen, exactly the opposite of what a human mission will provide.

What specifically would a crew do during a NEO visit? An astronaut on a planet typically would explore the surface, map geological relations where possible, collect representative samples of the units and rock types that can be discerned, and collect as much mapping and compositional data as possible to aid in the interpretation of the returned samples. In the case of a NEO, many of these activities would not be particularly fruitful. The asteroid is either a pile of rubble or a single huge boulder. Chondritic meteorites are uniform in composition, so geological setting is not particularly instructive. We do have questions about the processes of space weathering, the changes that occur in rocks as a result of their exposure to space for varying lengths of time. Such questions could be addressed by a simple robotic sample collector, as the recently approved OSIRIS mission plans to do.

One question that could be addressed by human visitors to asteroids is their internal make-up and structure. Some appear to be rubble piles while others are nearly solid – why such different fates in different asteroids? By using active seismometry (acoustic sounding), a human crew could lay out instruments and sensors to decipher the density profile of an asteroid. Understanding the internal structure of an asteroid is important for learning how strong such objects are; this could be an important factor in devising mitigation strategies in case we ever have to divert a NEO away from a collision trajectory with the Earth. As mentioned in my preceding post, the crew had better work quickly – loiter times at the asteroid will probably be short, on the order of a few days at most.

Although we can explore asteroids with human missions, it seems likely that few significant insights into the origins and processes of the early Solar System will result from such exploration. Such study is already a very active field, using the samples that nature has provided us – the meteorites. Sample collection from an asteroid will yield more samples of meteorites, only without the melted fusion crusts that passage through the Earth’s atmosphere creates. In other words, from this mission, scientific progress will be incremental, not revolutionary.

In contrast, because they yield information on geological histories and processes at planet-wide scales, sample collection and return from a large planetary body such as the Moon or Mars could revolutionize our knowledge of these objects in particular and the Solar System in general. Many years prior to the Moon missions, we had meteorites that showed impact metamorphic effects but the idea of impact-caused mass extinctions of life on Earth only came after we had fully comprehended the impact process recorded in the Apollo samples from the Moon. The significance of impact-related mineral and chemical features were not appreciated until we had collected samples with geological context to understand what the lunar samples were telling us.

Of course, science being unpredictable, some major surprise that could revolutionize our knowledge may await us on some distant asteroid. But such surprises doubtless await us in many places throughout the Solar System and the best way to assure ourselves that we will eventually find them is to develop the capability to go anywhere in space at any time. That means developing and using the resources of space to create new capabilities. I will consider that in my next post.